Marriage creates a strong bond between people, something to make it harder for them to break apart. When two people get married, they're committing to each other. It's hard to seperate, but that's the point, they want to have to try again before they leave each other. Without marriage, breakups can happen in a second.

Marriage is a beautiful thing between two people, who are there for each other no matter what. It's a way to show that someone means something to you.
It also makes it easier for couples to work through legal and financial things, as they are seen as a joint unit in the eyes of the law.